For sculpting, the easiest way to judge that would be to scale the model on your monitor to the largest it’ll be in normal game play. If it’s s first person game where the player will be using it as cover, it’ll need a lot of detail at eye level. If it’s a third person game, the camera won’t get anywhere near that close. If it’s for a top down game, it’ll be even smaller. You don’t want the model to look like a blob. Also if you are going to be layering in other textures and normal maps, the sculpting detail can be much lower, and you don’t need to worry about sculpting high frequency, or small, details. Just make sure you get the medium and large forms right.